Recently, as an inorganic photoconductive material, amorphous silicon, amorphous selenium, cadmium sulfide, zinc oxide, etc., are used, but some of these materials are expensive owing to the difficulty of the production thereof and some of them cause a problem from the view point of an environmental protection owing to the toxicity of them.
On the other hand, as an organic photoconductive material, in particular, a function-separating type light-sensitive material comprising a composition of a charge generating material and a charge transporting material has been positively proposed (e.g., U.S. Pat. No. 3,791,826). In the system, by using a material showing a high carrier generation efficiency as the charge generating material and combining the charge generating material and a material having a high charge transportability as a charge transporting material, there is a possibility of obtaining an electrophotographic photoreceptor having a high sensitivity.
In these materials, the charge transporting material is required to efficiently receive carriers generated in the charge generating material by the irradiation of light under the application of an electric field, quickly transport the carriers in the photoreceptor layer, and quickly erase the charge on the surface.
The transferring velocity of a carrier per unit electric field is called a carrier drift mobility. A high carrier drift mobility means that the carrier transfers quickly in the charge transporting layer.
The carrier drift mobility is specific to the charge transporting material, and hence in order to attain the high carrier drift mobility, it is necessary to use a material showing a high carrier drift mobility. The carrier drift mobility by the conventional materials has not yet reached a sufficient level at present.
On the other hand, since the carrier drift mobility depends upon the concentration of the charge transporting material, a method of increasing the concentration of a charge transporting material in a charge transporting layer is employed. The case that the concentration of a charge transporting material becomes the highest is the case that the charge transporting layer is formed by the charge transporting material only and such a charge transporting layer is formed by a vapor deposition method, etc. For example, an organic electroluminescence (EL) device, etc., is prepared by the method as described above [e.g., C. W. Tang and S. A. VanSlyke, Appl. Phys. Lett., 51, 913(1987)].
However, when the thin layer is a vapor-deposited layer composed of a charge transporting material only, in particular, crystals are liable to deposit and pin holes are liable to form, whereby it is difficult to form the layer having a uniform quality.
Also, when an organic solvent solution containing a charge transporting material at a high concentration together with a binder polymer is coated to form a coated layer, it is necessary to form a uniform organic thin layer having no deposition of crystals and no formation of pin holes. This is because since a high electric field is applied to the thin layer formed, if the thin layer has fine crystals or pin holes, a dielectric breakdown occurs at the positions of forming the fine crystals or pin holes to cause noise.
Also, even when the characteristics of both the charge generating material and the charge transporting material are good, it is important that the injection of carriers from the charge generating material into the charge transporting material, that is, the injection of carriers from the charge generating layer into the charge transporting layer be carried out with a good efficiency. The injection of the carriers depends upon the characteristics of the interface between a charge generating material (or a charge generating layer) and a charge transporting material (or a charge transporting layer) and hence the injection of the carriers varies between the kinds of the materials being used. As described above, various conditions are required for a charge transporting material.
Hitherto, as a charge transporting material, for example, a distyryl compound represented by the following formula (II) is proposed in JP-A-63-269158 (the term "JP-A" as used herein means an "unexamined published Japanese patent application": ##STR3## wherein Ar.sub.1 to Ar.sub.4 each independently represents an alkyl group, an aralkyl group, an aromatic hydrocarbon group which may have a substituent, or an aromatic heterocyclic group which may have a substituent and A represents an alkyl group, an aralkyl group, an aryl group or a heterocyclic group, wherein each group may have a substituent.
Also, JP-A-3-11355 discloses a distyryl compound represented by the following formula (III): ##STR4## wherein R.sup.3, R.sup.4, R.sup.8, and R.sup.9 each independently represents an alkyl group, an aralkyl group, or an aryl group which each may have a substituent; R.sup.5, R.sup.7, and R.sup.10 each independently represents a hydrogen atom, an alkyl group, an alkoxy group, or a halogen atom; R.sup.6 and R.sup.11 each represents an alkyl group or an alkoxy group; l and n each represents an integer of from 1 to 3; and m represents an integer of from 0 to 2.
However, in the compounds which were obtained in the Examples or were actually described in the specification, the dialkylamino group substituted on both terminals thereof is the same and there are problems that the compound is insufficient in the point of solubility in a binder polymer and even when the compound is dissolved in a binder polymer, when a film or layer is formed using it, crystallization occurs, pin holes form, and the film or layer is whitened or becomes brittle, which results in forming defects on the images formed, and hence there is a restriction on the addition amount of the compound.